This invention relates to tools and methods for preparing woodturning scrapers.
Traditional woodturning cutting tools fall generally into two categories: (1) scrapers and (2) chisels, gouges, parting tools and the like. Chisels, gouges, parting tools and the like generally have cutting edges formed by tool surfaces that meet at a relatively sharp angle. By contrast, turning scrapers have a working edge or arris or piend formed by tool surfaces that meet at a larger bevel angle. Such tools have traditionally been prepared for use by abrading or grinding the tool bevel in order to form a sharp but somewhat rough edge or burr where the bevel intersects the face of the tool.
This turning scraper working edge can be improved by lapping or honing to remove the burr and by then "burnishing" the bevel adjacent to the edge. In preparing woodworking "scrapers," "burnishing" is not merely polishing but is a process involving sliding the bevel or other tool surface being burnished in contact with a burnishing surface of smooth material that is harder than the metal from which the scraper is made, while the bevel surface and burnishing surface are pressed together with substantial force.
Best turning scraper burnishing results are achieved when the same burnishing angle (the angle between the burnishing surface and the bevel) is maintained throughout the burnishing process and the burnishing angle is as small as possible. This creates the widest possible burnishing contact, which acts to prevent the scraper from suddenly "catching" during the turning process. "Catching" occurs when the burnisher digs into the workpiece or quickly removes an undesirably large quantity of material from the workpiece and is frequently accompanied by oral commentary exploring the extremes of the turner's vocabulary.
However, the smaller the angle between the burnishing surface and the bevel, the greater the force required to deform the steel of the scraper in the process of forming or "turning" a hook on the scraper that is part of the intended result of burnishing the scraper. If the bevel grind angle is reduced, thereby creating a greater angle between the burnishing surface and the bevel and consequently making it easier to "turn" the edge to form a hook, the region of the bevel that is burnished is narrower, and the scraper is substantially more likely to "catch" or gouge the workpiece during use. Moreover, a hook formed with a greater burnishing angle is more fragile than one formed with a smaller burnishing angle and therefore will not last as long.
Some turners burnish turning scrapers to form a "hook" using a conventional burnishing rod, but it is difficult to control the burnishing angle during this process and it is very difficult to apply adequate pressure during burnishing in this manner.
Furthermore, in recent years most turning tools have begun to be manufactured of high speed steel, which is substantially more difficult to deform and therefore successfully burnish than softer steel alloys.